其实流程算起来也不算复杂，所以就直接用代码注释来做吧
这里边涉及这么几个方法：
1、public List<InputSplit> getSplits(JobContext job), 这个由客户端调用来获得当前Job的所有分片(split)，然后发送给JobTracker（新API中应该是ResourceManager），而JobTracker根据这些分片的存储位置来给TaskTracker分配map任务去处理这些分片。这个方法用到了后边的listStatus，然后根据得到的这些文件信息，从FileSystem那里去拉取这些组成这些文件的块的信息(BlockLocation)，使用的是getFileBlockLocation(file,start,len)，这个方法是与使用的文件系统实现相关的(FileSystem，LocalFileSystem，DistributedFileSystem)
/**
* Generate the list of files and make them into FileSplits.
*/
public List<InputSplit> getSplits(JobContext job
) throws IOException {
long minSize = Math.max(getFormatMinSplitSize(), getMinSplitSize(job));
long maxSize = getMaxSplitSize(job);
// generate splits
List<InputSplit> splits = new ArrayList<InputSplit>();
List<FileStatus>files = listStatus(job); //2
for (FileStatus file: files) {
Path path = file.getPath();
FileSystem fs = path.getFileSystem(job.getConfiguration());
long length = file.getLen();
/*Return an array containing hostnames, offset and size of portions of the given
file. For a nonexistent file or regions, null will be returned. This call is
most helpful with DFS, where it returns hostnames of machines that contain the
given file. The FileSystem will simply return an elt containing 'localhost'.*/
BlockLocation[] blkLocations = fs.getFileBlockLocations(file, 0, length);
if ((length != 0) && isSplitable(job, path)) {
long blockSize = file.getBlockSize();
long splitSize = computeSplitSize(blockSize, minSize, maxSize); //3
long bytesRemaining = length;
while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining); //4
splits.add(new FileSplit(path, length-bytesRemaining, splitSize,
blkLocations[blkIndex].getHosts()));
bytesRemaining -= splitSize;
}
if (bytesRemaining != 0) {
splits.add(new FileSplit(path, length-bytesRemaining, bytesRemaining,
blkLocations[blkLocations.length-1].getHosts()));
}
} else if (length != 0) {
//这里使用的是FileSplit，在RecordReader实现中拿到Split的时候就可以向下转型，从而拿到一些分片的信息
splits.add(new FileSplit(path, 0, length, blkLocations[0].getHosts()));
//Create empty hosts array for zero length files
splits.add(new FileSplit(path, 0, length, new String[0]));
}
}
// Save the number of input files in the job-conf
job.getConfiguration().setLong(NUM_INPUT_FILES, files.size());
LOG.debug("Total # of splits: " + splits.size());
return splits;
}
2、protected List<FileStatus> listStatus(JobContext job), 先根据“mapred.input.dir”的配置值去得到用户指定的所有Path。然后根据这个JobContext的Configuration得到FileSystem(当然，更可能是 DistributedFileSystem )。最后应用用户可能设置了的PathFilter，通过FileSystem获取所有这些Path所代表的File(FileStatus)。注：这个方法的东西相当多，很多内容还十分陌生。
/** List input directories.
* Subclasses may override to, e.g., select only files matching a regular
* expression.
*
* @param job the job to list input paths for
* @return array of FileStatus objects
* @throws IOException if zero items.
*/
protected List<FileStatus> listStatus(JobContext job
) throws IOException {
List<FileStatus> result = new ArrayList<FileStatus>();
Path[] dirs = getInputPaths(job);
if (dirs.length == 0) {
throw new IOException("No input paths specified in job");
}
// get tokens for all the required FileSystems..
TokenCache.obtainTokensForNamenodes(job.getCredentials(), dirs,
job.getConfiguration());
List<IOException> errors = new ArrayList<IOException>();
// creates a MultiPathFilter with the hiddenFileFilter and the
// user provided one (if any).
List<PathFilter> filters = new ArrayList<PathFilter>();
filters.add(hiddenFileFilter);
PathFilter jobFilter = getInputPathFilter(job);
if (jobFilter != null) {
filters.add(jobFilter);
}
PathFilter inputFilter = new MultiPathFilter(filters);
for (int i=0; i < dirs.length; ++i) {
Path p = dirs[i];
FileSystem fs = p.getFileSystem(job.getConfiguration());
FileStatus[] matches = fs.globStatus(p, inputFilter);
if (matches == null) {
errors.add(new IOException("Input path does not exist: " + p));
} else if (matches.length == 0) {
errors.add(new IOException("Input Pattern " + p + " matches 0 files"));
} else {
for (FileStatus globStat: matches) {
//可以看到，这里没有对遍历进行递归，但是在权威指南里边提到可以通过设置mapred.input.dir.recursive来
//修改这一行为，暂时还不太清楚是如何实现的，在项目中查找这一字符串应该能找到一些线索
if (globStat.isDir()) {
for(FileStatus stat: fs.listStatus(globStat.getPath(),
inputFilter)) {
result.add(stat);
}
} else {
result.add(globStat);
}
}
}
}
if (!errors.isEmpty()) {
throw new InvalidInputException(errors);
}
LOG.info("Total input paths to process : " + result.size());
return result;
}
3、protected long computeSplitSize(long blockSize, long minSize, long maxSize),计算出当前Job所配置的分片最大尺寸。
protected long computeSplitSize(long blockSize, long minSize,
long maxSize) {
return Math.max(minSize, Math.min(maxSize, blockSize));
}
4、protected int getBlockIndex(BlockLocation[] blkLocations, long offset), 由于组成文件的块的信息已经获得了，只需要根据offset来计算所在的那个块就行了。
protected int getBlockIndex(BlockLocation[] blkLocations,
long offset) {
for (int i = 0 ; i < blkLocations.length; i++) {
// is the offset inside this block?
if ((blkLocations[i].getOffset() <= offset) &&
(offset < blkLocations[i].getOffset() + blkLocations[i].getLength())){
return i;
}
}
BlockLocation last = blkLocations[blkLocations.length -1];
long fileLength = last.getOffset() + last.getLength() -1;
throw new IllegalArgumentException("Offset " + offset +
" is outside of file (0.." +
fileLength + ")");
}